Radio communication systems rely on modulating carrier frequencies in a finite portion of the electromagnetic spectrum to wirelessly transmit and receive signals. Modulation can be performed on the amplitude, frequency, and/or phase of the carrier frequency to separate the signal from unwanted noise. The signals typically convey information such as voice, video, and computer data to and from transceiving devices such as cellular base stations, cellular subscriber units, and personal computers.
The portion of the electromagnetic spectrum occupied by a particular transmission or communication system (i.e. bandwidth) may be wide or narrow. Wideband signals can be used to transmit large amounts of data in a relatively short period of time. For example, large computer data files and real-time video could benefit from a wideband signal. Narrow-band signals can be used to conserve the electromagnetic spectrum when transmitting signals with more modest requirements. For example, base stations and cellular subscriber units in most conventional cellular communication systems transmit and receive voice signals using a relatively narrow-band signal.
The amount of usable electromagnetic spectrum is limited by technology, environment, and cost. Extremely high frequency signals require expensive transceiving equipment. Accordingly, communication systems benefit by sharing desirable frequencies. Well known multiple access techniques, such as code division multiple access (CDMA), time division multiple access (TDMA), and frequency division multiple access (FDMA) can be used by a communication system to share the electromagnetic spectrum available to that system.
Spread spectrum communication systems transmit signals occupying a bandwidth in excess of the minimum necessary to send the information. Spreading may be accomplished by means of a code which is independent of the data. Each symbol of the spreaded signal is a chip. The chip rate determines the bandwidth of the signal; and, the ratio of the chip rate to the incoming information data rate is the spreading gain. In the mobile communication industry, a redundancy code such as a forward error correction code is also generally included when computing the total spreading gain.
In spread spectrum CDMA systems a predefined chip rate (i.e. one bandwidth) is typically used so that orthogonality can be readily achieved using binary orthogonal codes (e.g., Walsh codes) thus minimizing intra-cell interference. The chip rate is the rate (i.e., frequency) at which changes (i.e., modulations) are being made to the carrier frequency. There are many reasons for spreading the spectrum. One application in a mobile communications environment is to achieve efficient multiple access (i.e., CDMA). By spreading the signal to wider bandwidth, CDMA allows multiple users to share the same frequency band at the same time. More conventional applications for spread spectrum communications include anti-jamming, anti-interference, and low probability of intercept.
Prior art approaches to bandwidth utilization suffer from certain drawbacks. For instance, prior art approaches do not allow wideband signals to occupy excess capacity in a narrow band system or narrow-band signals to occupy excess capacity in a wide band system. Further, prior art approaches require new communications systems infrastructure (e.g., base stations) to support new types of signals (i.e., signals using different bandwidths).
As known in the art, a wide-band spreaded signal can in principal overlay one or more narrow-band signals that are transmitted simultaneously with the wideband signal. Spreaded signals occupy a bandwidth that is wider than necessary for their transmission, thereby spreading their total power across a wideband spectrum with respect to narrow-band underlay signals. Ideally, narrow-band receivers recognize the portion of the spreaded signal within their narrow spectrum as noise and can discriminate their narrow-band signals from the interfering wideband signals.
In practice, simple spreading of a wideband signal fails to provide a workable overlay solution. For example, in the cellular/data system discussed above, a spreaded wideband transmission on the forward link (i.e. cell site to mobile station) introduces sufficient spectral power within the underlay narrow-band spectrum so that interference becomes intolerable, or discrimination at the narrow-band mobile user's units becomes cost prohibitive or infeasible.